A design, adaptive to the flow velocity, is developed for a hydrokinetic energy converter based on an oscillator undergoing Flow Induced Vibrations (FIVs); primarily Vortex Induced Vibrations (VIV) and galloping. This Alternating-Lift Technology implements a nonlinear spring to passively optimize the oscillator. The FIV of a single, rigid, circular cylinder, with distributed surface roughness, suspended on end-springs with piecewise linear continuous restoring force, are studied for Reynolds number 24,000 ≤ Re ≤ 120,000. Linear viscous damping for energy harnessing and different piecewise linear spring functions are used as parameters. The harnessed power envelope is established based on the results of linear spring stiffness and two nonlinear piecewise stiffness functions. Selective roughness is applied to enhance FIV and increase the hydrokinetic energy captured by the converter at higher Reynolds numbers. The second generation of a virtual spring-damping system (Vck), developed in the Marine Renewable Energy Laboratory (MRELab), enables embedded, real-time, computer-controlled change of viscous damping and spring stiffness for fast implementation of oscillator particulars. Experimental results for amplitude response, frequency response, energy harvesting, and efficiency are presented and discussed. All experiments are conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) Each nonlinear, piecewise-linear, stiffness function has its own merits in power harnessing; the differences lie in the FIV characteristics of the different regions of the flow. (2) The nonlinear spring converter can harness energy from flows as low as 0.275 m/s with no upper limit. (3) The new adaptive function exhibits higher harnessed power in the upper VIV branch, transition from VIV to galloping, and fully developed galloping regions. (4) The FIV response is predominantly periodic for all nonlinear spring functions used. (5) Optimal power harnessing is achieved by changing the nonlinear piecewise spring function and the linear viscous damping. (6) The optimally harnessed power envelope is established according to the experimental results of the linear and nonlinear stiffness springs; four performance zones are established based on FIV.